A Blog About the Science and Sociology of Cosmology and Dark Matter

Case Western Reserve University

Stacy McGaugh

I was raised to believe that it was rude to tell people

 I told you so

. Yet that’s pretty much the essence of the scientific method: we test hypotheses by making predictions, then checking to see which told us the correct result in advance of the experiment. So: I told you so. Our paper on massive galaxies at high redshift is out in the Astrophysical Journal today.

Massive Galaxies at High Redshift: we told you so

There is a rule of thumb in scientific publication that if a title is posed a question, the answer is no.  It sucks being so far ahead of the field that I get to watch people repeat the mistakes I made (or almost made) and warned against long ago. There have been persistent claims of deviations of one sort or another from the Baryonic Tully-Fisher relation (BTFR). So far, these have all been obviously wrong, for reasons we’ve discussed before.

Are there credible deviations from the baryonic Tully-Fisher relation?

After writing the commentary on the latest fin du MOND, it occurred to me that there are many issues that I consider to be obvious. But I’ve been thinking about them for a quarter century, so perhaps they may need to be clearly elucidated for those who don’t share that background. I am thinking, in particular, of galaxy formation modelers and theorists.

The Grand Observational Challenge for Galaxy Formation Simulations

Given recent developments in the long-running hunt for dark matter and the difficulty interpreting what this means, it seems like a good juncture to re-up* this:  The history of science is a decision tree. Vertices appear where we must take one or another branching. Sometimes, we take the wrong road for the right reasons. A good example is the geocentric vs. heliocentric cosmology.

Decision Trees &amp; Philosophical Blunders

I’m just back from the World Science Festival where I participated in the discussion shaking up the dark universe. It was an interesting experience, and mostly positive despite some offstage diva behavior. One thing it drove home to me was how hard it is to communicate across a gulf of different training and experience. Not just from scientists to the public, but between scientists.

New York’s all right if you like saxophones

There has been some debate of late over the role of falsifiability in science. Falsifiability is the philosophical notion advocated by Popper as an acid test to distinguish between ideas that are scientific and those that are not. In short, for a theory to be scientific, it has to be subject to falsification.

Falsifiability and Persuadability in Science

People often ask for a straight up comparison between ΛCDM and MOND. This is rarely possible because the two theories are largely incommensurable. When one is eloquent the other is mute, and vice-versa. It is possible to attempt a comparison about how bad the missing baryon problem is in each.

Missing baryons in LCDM and MOND

I think the time has come for another update on wide binaries. These were intensely debated at the conference in St. Andrews, with opposing camps saying they did or did not show MONDian behavior. Two papers by independent authors have recently been refereed and published: Chae (2023) in the

 Astrophysical Journal

and Hernandez (2023) in

 Monthly Notices

. These papers both find evidence for MONDian behavior in wide binaries.

Wide Binary Results Favoring MOND

In order to agree on an interpretation, we first have to agree on the facts. Even when we agree on the facts, the available set of facts may admit multiple interpretations.

Common ground

The Milky Way and its nearest giant neighbor Andromeda (M31) are surrounded by a swarm of dwarf satellite galaxies. Aside from relatively large beasties like the Large Magellanic Cloud or M32, the majority of these are the so-called dwarf spheroidals. There are several dozen examples known around each giant host, like the Fornax dwarf pictured above.

Dwarf Satellite Galaxies and Low Surface Brightness Galaxies in the Field. I.

OK, I’m not even going to try to answer that one. But I am going to do some comparison exploration. A complaint often leveled against MOND is that it is not a theory. Or not a complete theory. Or somehow not a proper one. Sometimes people confuse MOND with the empirical observations that display MONDian phenomenology. I would say that MOND is a hypothesis, as is dark matter.

What is theory?

With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.

– John von Neumann The simple and elegant cosmology encapsulated by the search for two numbers has been replaced by ΛCDM. This is neither simple nor elegant. In addition to the Hubble constant and density parameter, we now also require distinct density parameters for baryonic mass, non-baryonic cold dark matter, and dark energy.

And then there were six

I said I would occasionally talk about scientific papers. Today’s post is about the new paper

 Testing Feedback-Modified Dark Matter Haloes with Galaxy Rotation Curves: Estimation of Halo Parameters and Consistency with ΛCDM

by Harley Katz et al. I’ve spent a fair portion of my career fitting dark matter halos to rotation curves, and trying to make sense of the results.

Dark Matter halo fits – today’s cut

I was on vacation last week. As soon as I got back, the first thing I did was fall off my bike onto a tree stump, breaking my wrist. I’ll be okay, but I won’t be typing a lot. This post is being dictated to software; I hope I don’t have to do too much editing. I let the software generate the image above based on the prompt “dark matter properties illustrated” and I don’t think we should hold our breath for AI to help us out with this.

Required dark matter properties

When I wrote about Nobel prizes a little while back, I did not expect to return to the subject.

A Nobel prize in physics for something that is not physics

I should perhaps explain a little about the title of the last post. It is perfectly obvious to me. But probably not to anyone else. Our brains work in subtly different ways. One thing that mine does, whether I like it or not, is memorize lines and make obscure links between them.

Plate of Shrimp

In the series of recent posts I’ve made about the Milky Way, I missed an important reply made in the comments by Francois Hammer, one of the eminent scientists doing the work. I was on to writing the next post when he wrote it, and simply didn’t see it until yesterday. Dr. Hammer has some important things to say that are both illustrative of the specific topic and also of how science should work.

A Response to Recent Developments Concerning the Gravitational Potential of the Milky Way

There are two basic approaches to cosmology: start at redshift zero and work outwards in space, or start at the beginning of time and work forward. The latter approach is generally favored by theorists, as much of the physics of the early universe follows a “clean” thermal progression, cooling adiabatically as it expands.

The next cosmic frontier: 21cm absorption at high redshift

I have been musing for a while on the idea of writing about Naturalness in science, particularly as it applies to the radial acceleration relation. As a scientist, the concept of Naturalness is very important to me, especially when it comes to the interpretation of data. When I sat down to write, I made the mistake of first Googling the term. The top Google hits bear little resemblance to what I mean by Naturalness.

What is Natural?

I happened to visit this blog as a visitor from a computer not mine. Seeing it that way made me realize how obnoxious the ads had become. So WordPress’s extortion worked; I’ve agreed to send them a few $ every month to get rid of the ads. With it comes a new domain name: tritonstation.com. Bookmarks to the previous website (tritonstation.wordpress.com) should redirect here. Let me know if a problem arises, or the barrage of ads fails to let up.

New web domain

This Thanksgiving, I’d highlight something positive. Recently, Bob Sanders wrote a paper pointing out that gas rich galaxies are strong tests of MOND. The usual fit parameter, the stellar mass-to-light ratio, is effectively negligible when gas dominates. The MOND prediction follows straight from the gas distribution, for which there is no equivalent freedom.

Hypothesis testing with gas rich galaxies

I am a white American male. As such, I realize that there is no way for me to grasp and viscerally appreciate all the ways in which racism afflicts black Americans. Or, for that matter, all the ways in which sexism afflicts women. But I can acknowledge that these things exist. I can recognize when it happens. I’ve seen it happen to others, both friends and strangers. I can try not to be part of the problem.

The Other

Kuhn noted that as paradigms reach their breaking point, there is a divergence of opinions between scientists about what the important evidence is, or what even counts as evidence. This has come to pass in the debate over whether dark matter or modified gravity is a better interpretation of the acceleration discrepancy problem. It sometimes feels like we’re speaking about different topics in a different language.

What we have here is a failure to communicate

Dark matter remains undetected in the laboratory. This has been true for forever, so I don’t know what drives the timing of the recent spate of articles encouraging us to keep the faith, that dark matter is still a better idea than anything else. This depends on how we define “better.”  There is a long-standing debate in the philosophy of science about the relative merits of accommodation and prediction.

Define “better”

At the dawn of the 21st century, we were pretty sure we had solved cosmology. The Lambda Cold Dark Matter (LCDM) model made strong predictions for the power spectrum of the Cosmic Microwave Background (CMB). One was that the flat Robertson-Walker geometry that we were assuming for LCDM predicted the location of the first peak should be at ℓ = 220.

Second peak bang on

I read somewhere – I don’t think it was Kuhn himself, but someone analyzing Kuhn – that there came a point in the history of science where there was a divergence between scientists, with different scientists disagreeing about what counts as a theory, what counts as a test of a theory, what even counts as evidence. We have reached that point with the mass discrepancy problem.

Divergence

In 1984, I heard Hans Bethe give a talk in which he suggested the dark matter might be neutrinos. This sounded outlandish – from what I had just been taught about the Standard Model, neutrinos were massless. Worse, I had been given the clear impression that it would screw everything up if they did have mass. This was the pervasive attitude, even though the solar neutrino problem was known at the time. This did not compute!

Neutrinos got mass!

A quick note to put the acceleration discrepancy in perspective. The acceleration discrepancy, as Bekenstein called it, more commonly called the missing mass or dark matter problem, is the deviation of dynamics from those of Newton and Einstein. The quantity D is the amplitude of the discrepancy, basically the ratio of total mass to that which is visible.

Astronomical Acceleration Scales

I wrote my own recollection of Vera Rubin recently. Her long time home institution, the Department of Terrestrial Magnetism (DTM) of the Carnegie Institution of Washington recently held a lunch in her honor. Unfortunately my travel schedule precluded me from attending. However, they have put together a wonderful website that I recommend to everyone.

DTM’s Remembering Vera

A long standing problem in cosmology is that we do not have a full accounting of all the baryons that we believe to exist. Big Bang Nucleosynthesis (BBN) teaches us that the mass density in normal matter is Ω

 b

≈ 5%. One can put a more precise number on it, but that’s close enough for our purposes here. Ordinary matter fails to account for the closure density by over an order of magnitude.

Missing Baryons

Flat rotation curves were the first clear evidence that the dynamics of galaxies do not follow the same rules as planetary systems. But they do follow rules. These include asymptotic flatness, Tully-Fisher, the luminosity-size-rotation curve shape relation (aka the `universal’ rotation curve), Renzo’s rule, and the central density relation.

The Third Law of Galactic Rotation

Who we give prizes to is more a matter of sociology than science. Good science is a prerequisite, but after that it is a matter of which results we value in the here and now. Results that are guaranteed to get a Nobel prize, like the detection of dark matter, attract many suitors who pursue them vigorously. Results that come as a surprise can be more important than the expected results, but it takes a lot longer to recognize and appreciate them.

Sociology in the hunt for dark matter

Some people have asked me to comment on the

 Scientific American

article

 What if We Never Find Dark Matter?

by Slatyer &amp; Tait. For the most part, I find it unobjectionable – from a certain point of view.

What if we never find dark matter?

The time is approaching when Nobel prizes are awarded. This inevitably leads to a lot of speculation and chattering rumor. Last year one publication, I think it was

 Physics Today

, went so far as to publish a list of things various people thought should be recognized. This aspirational list was led, of course, by dark matter.

Nobel prizes that were, that might have been, and others that have not yet come to pass

I want to take another step back in perspective from the last post to say a few words about what the radial acceleration relation (RAR) means and what it doesn’t mean. Here it is again:  This information was not available when the dark matter paradigm was developed. We observed excess motion, like flat rotation curves, and inferred the existence of extra mass. That was perfectly reasonable given the information available at the time.

Why’d it have to be MOND?

Last time, we discussed the remarkable result that gravitational lensing extends the original remarkable result of flat rotation curves much farther out, as far as the data credibly probe. This corroborates and extends the result of Brouwer et al. They did a thorough job, but one thing they did not consider was Tully-Fisher.

Tully-Fisher from gravitational lensing

In the previous post, we discussed how lensing data extend the Radial Acceleration Relation (RAR) seen in galaxy kinematics to very low accelerations. Let’s zoom out now, and look at things at higher accelerations and from a historical perspective.

The Radial Acceleration Relation starting from high accelerations

We will return to our usual programming shortly. But first, a few words on the eclipse experience last Monday.

 
 It. Was. Awesome
 

. That’s a few words, so Mission Accomplished. That’s really all I had planned to say. However, I find I am still giddy from this momentous event, so will share my experience of the day, such as words can humbly convey.

The Eclipse Experience

Yes, some. That much is a step forward from a decade ago, when a common assumption was that the Milky Way’s rotation curve remained flat at the speed at which the sun orbited. This was a good guess based on empirical experience with other galaxies, but not all galaxies have rotation curves that are completely flat, nor can we be sure the sun is located where that is the case.

Is the Milky Way’s rotation curve declining?

I just got back from a visit to the Carnegie Institution of Washington where I gave a talk and saw some old friends. I was a postdoc at the Department of Terrestrial Magnetism (DTM) in the ’90s. DTM is so-named because in their early days they literally traveled the world mapping the magnetic field. When I was there, DTM

 +

had a small extragalactic astronomy group including Vera Rubin*, Francois Schweizer, and John Graham.

It is not linear

As predicted, JWST has been seeing big galaxies at high redshift. There are now many papers on the subject, ranging in tone from “this is a huge problem for LCDM” to “this is not a problem for LCDM at all” – a dichotomy that persists. So – which is it?  It will take some time to sort out. There are several important aspects to the problem, one of which is agreeing on what LCDM actually predicts.

Quantifying the excess masses of high redshift galaxies

Screw the Earth and its smoking habit. The end of 2023 approaches, so let’s talk about the whole universe, which is its own special kind of mess. As I’ve related before, our current cosmology, LCDM, was established over the course of the 1990s through a steady drip, drip, drip of results in observational cosmology – what Peebles calls the classic cosmological tests. There were many

 contributory

results;

Holiday Concordance

People often ask me of how “perfect” MOND has to be. The short answer is that it agrees with galaxy data as “perfectly” as we can perceive – i.e., the scatter in the credible data is accounted for entirely by known errors and the expected scatter in stellar mass-to-light ratios. Sometimes it nevertheless looks to go badly wrong. That’s often because we need to know both the mass distribution and the kinematics perfectly.

How things go mostly right or badly wrong

Continuing from last time, let’s compare recent rotation curve determinations from Gaia DR3:  These are different analyses of the same dataset. The Gaia data release is immense, with billions of stars. There are gazillions of ways to parse these data. So it is reasonable to have multiple realizations, and we shouldn’t expect them to necessarily agree perfectly: do we look exclusively at K giants? A stars?

Recent Developments Concerning the Gravitational Potential of the Milky Way.&nbsp;II. A Closer Look at the Data

Alert reader Dan Baeckström recently asked about NGC 1277, as apparently some people have been making this out to be some sort of death knell for MOND.  My first reaction was NGC who? There are lots of galaxies in the New General Catalog (new in 1888, even then drawing heavily on earlier work by the Herschels). I’m well acquainted with many individual galaxies, and can recall many dozens by name, but I do not know every single thing in the NGC.

Is NGC 1277 a problem for MOND?

Last time, I commented on the developing situation with binary stars as a test of MOND. I neglected to enable comments for that post, so have done so now. Indranil Banik has shared his perspective on wide binaries in a talk on the subject that is available on Youtube, included below. Indranil and his collaborators are not seeing a MOND effect in wide binaries. Others have, as I discussed in the previous post.

Commentary on Wide Binaries

I want to start by thanking those of you who have contributed to maintaining this site. This is not a money making venture, but it does help offset the cost of operations. The title is not related to this, but rather to a flood of papers addressing the questions posed in recent posts. I was asking last time “take it where?” because it is hard to know what cosmology under UT will look like. In particular, how does structure formation work?

Ask and receive

Imagine if you are able that General Relativity (GR) is correct yet incomplete. Just as GR contains Newtonian gravity in the appropriate limit, imagine that GR itself is a limit of some still more general theory that we don’t yet know about. Let’s call it Underlying Theory (UT) for short.

Imagine if you can

Reality check   Before we can agree on the interpretation of a set of facts, we have to agree on what those facts are. Even if we agree on the facts, we can differ about their interpretation. It is OK to disagree, and anyone who practices astrophysics is going to be wrong from time to time. It is the inevitable risk we take in trying to understand a universe that is vast beyond human comprehension.

In 1986, I was a grad student at Princeton, working in the atomic physics lab of Will Happer. It was at a department colloquium that I first heard a science talk that raised serious concerns about our use of fossil fuels potentially impacting the climate. This was not received well. People asked all sorts of questions, with much of the discussion revolving around feedback effects.

Cop28 president not even trying to hide his obvious bias

I have tried very hard to remain objective and even handed, but I find that I weary of the wide binary debate. I don’t know what the right answer will turn out to be. But I do have opinions. For starters, it is a big Galaxy. There is just too much to know. When I wrote about the Milky Way earlier this year, the idea was to set up an expectation value for wide binaries in the solar neighborhood.

A post in which some value judgements are made about the situation with wide binaries

Just as I was leaving for a week’s vacation, the dark matter search experiment LZ reported its first results. Now that I’m back, I see that I didn’t miss anything. Here is their figure of merit:  LZ is a merger of two previous experiments compelled to grow still bigger in the never-ending search for dark matter.

LZ: another non-detection

Cosmology used to be called the hunt for two numbers. It was simple and elegant. Nowadays we need at least six. It is neither simple nor elegant. So how did we get here? The two Big Numbers are, or at least up till the early-90s were, the Hubble constant H

 0

and the density parameter Ω. These told us Everything. Or so we thought. The Hubble constant is the expansion rate of the universe.

Two Numbers

Vera Rubin passed away a few weeks ago. This was not surprising: she had lived a long, positive, and fruitful life, but had faced the usual health problems of those of us who make it to the upper 80s. Though news of her death was not

 surprising

, it was deeply saddening. It affected me more than I had anticipated, even armed with the intellectual awareness that the inevitable must be approaching.

Ode to Vera

A recently discovered dwarf galaxy designated NGC1052-DF2 has been in the news lately. Apparently a satellite of the giant elliptical NGC 1052, DF2 (as I’ll call it from here on out) is remarkable for having a surprisingly low velocity dispersion for a galaxy of its type. These results were reported in Nature last week by van Dokkum et al., and have caused a bit of a stir.  It is common for giant galaxies to have some dwarf satellite galaxies.

The dwarf galaxy NGC1052-DF2

Virginia, your little friends are wrong. They have been affected by the skepticism of a skeptical age. They do not believe except they see. They think that nothing can be which is not comprehensible by their little minds. All minds, Virginia, whether they be men’s or children’s, are little.

Yes, Virginia, there is a Dark Matter

There is a new article in Science on the expansion rate of the universe, very much along the lines of my recent post. It is a good read that I recommend. It includes some of the human elements that influence the science. When I started this blog, I recalled my experience in the ’80s moving from a theory-infused institution to a more observationally and empirically oriented one.

Hubble constant redux

I was contacted today by a colleague at NASA’s Goddard Space Flight Center who was seeking to return some photographic plates of Halley’s comet that had been obtained with the Burrell Schmidt telescope. I at first misread the email – I get so many requests for data, I initially assumed that he was

 looking

for said plates.

Fuzzy Thing!

Cosmology is challenged at present by two apparently unrelated problems: the apparent formation of large galaxies at unexpectedly high redshift observed by JWST, and the tension between the value of the Hubble constant obtained by traditional methods and that found in multi-parameter fits to the acoustic power spectrum of the cosmic microwave background (CMB).  Maybe they’re not unrelated?

Early Galaxy Formation and the Hubble Constant Tension

Do not be too proud of this technological terror you’ve constructed. The ability to simulate the formation of large scale structure is insignificant next to the power of the Force.

– Darth Vader, Lord of the Sith The now standard cosmology, ΛCDM, has a well developed cosmogony that provides a satisfactory explanation of the formation of large scale structure in the universe.

Structure Formation Mythology

So I’m back from this small, convivial meeting. Many thanks to hosts Priya Natarajan and Doug Finkbeiner for putting the program together. I find it especially useful when scientists working on the same problem from different fields come together in this fashion.  It provides fresh perspective. I had wondered whether we were capable of genuine rethinking.

Some rethinking

A beginning is the time for taking the most delicate care that the balances are correct. … take care that you first place him in his time… And take the most special care that you locate [him] in his place…

– Princess Irulan (

 Dune

) A human being is complex beyond the knowledge of any other individual. Identifying oneself is not a simple matter. Professionally, I am a scientist.

Who am I?

I’ve been busy. There is a lot I’d like to say here, but I’ve been writing the actual science papers. Can’t keep up with myself, let alone everything else. I am prompted to write here now because of a small rant by Maury Goodman in the neutrino newsletter he occasionally sends out. It resonated with me.  First, some context. Neutrinos are particles of the Standard Model of particle physics.

The neutrino mass hierarchy and cosmological limits on their mass

I have been busy teaching cosmology this semester. When I started on the faculty of the University of Maryland in 1998, there was no advanced course on the subject. This seemed like an obvious hole to fill, so I developed one.

Cosmology, then and now

Vincent: Want to talk about MOND? Jules: No man, I don’t consider MOND. Vincent: Are you biased? Jules: Nah, I ain’t biased, I just don’t dig MOND, that’s all. Vincent: Why not? Jules: MOND is an ugly theory. I don’t consider ugly theories. Vincent: MOND makes predictions that come true. Fits galaxy data gooood.

{.alignnone .size-full .wp-image-3114 loading=“lazy” attachment-id=“3114” permalink=“http://tritonstation.com/2016/11/12/pulp-science/g1_pulp_fiction/” orig-file=“https://tritonstation.files.wordpress.com/2016/11/g1_pulp_fiction.jpg” orig-size=“800,340” comments-opened=“1”

Pulp Science

Galaxies are gravitationally bound condensations of stars and gas in a mostly empty, expanding universe. The tens of billions of solar masses of baryonic material that comprise the stars and gas of the Milky Way now reside mostly within a radius of 20 kpc. At the average density of the universe, the equivalent mass fills a spherical volume with a comoving radius a bit in excess of 1 Mpc.

Galaxy Formation – a few basics

Flat rotation curves and the Baryonic Tully-Fisher relation (BTFR) both follow from the Radial Acceleration Relation (RAR). In Mistele et al. (2024b) we emphasize the exciting aspects of the former; these follow from the RAR in the Mistele et al. (2024a). It is worth understanding the connection. First, the basic result:  The RAR of weak lensing extends the RAR from kinematics to much lower accelerations.

The Radial Acceleration Relation to very low accelerations

Mass is a basic quantity. How much stuff does an astronomical object contain?

Galaxy Stellar and Halo Masses: tension between abundance matching and kinematics

This clickbait title is inspired by the clickbait title of a recent story about high redshift galaxies observed by JWST. To speak in the same vernacular: LOL! What they mean, as I’ve discussed many times here, is that it is difficult to explain these observations in LCDM. LCDM does not encompass all of science.

 Science

* predicted exactly this.

Can’t be explained by science!

“Galaxies are made of stars.” Bob Schommer, quoted by Dave Silva in his dissertation on stellar populations This tongue-in-cheek quote is a statement of the obvious, at least for the 90+ years since Hubble established that galaxies are stellar systems comparable to and distinct from the Milky Way.

This tongue-in-cheek quote is a statement of the obvious, at least for the 90+ years since Hubble established that galaxies are stellar systems comparable to and distinct from the Milky Way. There’s interstellar gas and dust too, and I suppose for nearly half that time, people have also thought galaxies to be composed of dark matter. But you can’t see that;

A Stellar Population Mystery in a Low Surface Brightness Galaxy

Big galaxies at high redshift! That’s my prediction, anyway. A little context first. New Year, New Telescope   First, JWST finally launched. This has been a long-delayed NASA mission; the launch had been put off so many times it felt like a living example of Zeno’s paradox: ever closer but never quite there.

What JWST will see

There has already been one very quick attempt to match ΛCDM galaxy formation simulations to the radial acceleration relation (RAR). Another rapid preprint by the Durham group has appeared. It doesn’t do everything I ask for from simulations, but it does do a respectable number of them. So how does it do?  First, there is some eye-rolling language in the title and the abstract.

Another quick-trick simulation result

I find that my scientific colleagues have a variety of attitudes about what counts as a theory. Some of the differences amount to different standards. Others are simply misconceptions about specific theories. This comes up a lot in discussions of MOND. Before we go there, we need to establish some essentials. What is empirical?   I consider myself to be a very empirically-minded scientist. To me, what is empirical is what the data show.  Hmm.

What is empirical?

Predictive power in science isn’t everything. It’s the only thing.

This is a wise truth that has often been poorly interpreted. I despise some of the results that this sports quote has had in American culture. It has fostered a culture of bad sportsmanship in some places: an acceptance, even a dictum, that the ends justify the means – up to and including cheating, provided you can get away with it.  Winning every time is an impossible standard.

Predictive Power in Science

I travel to Cambridge, MA tomorrow to participate in the workshop Rethinking the Dark Matter Paradigm (I had nothing to do with the choice of title). I went to college at MIT in the ’80s, so is a bit back to the future for me in space as well as time. There is a lot to rethink, or nothing at all, depending on who you ask.

Rethinking the Dark Matter Paradigm

There has been some hand-wringing of late about the tension between the value of the expansion rate of the universe – the famous Hubble constant, H

 0

, measured directly from observed redshifts and distances, and that obtained by multi-parameter fits to the cosmic microwave background.

Tension in the Hubble constant

So the radial acceleration relation is a new law of nature. What does it mean? One reason we have posed it as a law of nature is that it is interpretation-free. It is a description of how nature works – in this case, a rule for how galaxies rotate.

 Why

nature behaves thus is another matter.

Four Strikes

Tell me, O muse, of that ingenious scientist who travelled far and wide after he had falsified the famous cosmology of Ptolemy. Many paradigms did he visit, and many were the theories with whose manners and customs he was acquainted; moreover he suffered much from the two body problem while trying to save the soul of science and raise his family safely at home.

Hello, World

I have not posted here in a while. This is mostly due to the fact that I have a job that is both engaging and demanding. I started this blog as a way to blow off steam, but I realized this mostly meant ranting about

 those fools at the academy!

of whom there are indeed plenty.

The kids are all right, but they can’t interpret a graph

I have said I wasn’t going to attempt to teach an entire graduate course on galaxy dynamics in this forum, and I’m not. But I can give some pointers for those who want to try it for themselves. It also provides some useful context for fans of Deur’s approach. The go-to textbook for this topic is

 Galactic Dynamics

by Binney &amp; Tremaine. The first edition was published in 1987, conveniently when I switched to grad school in astronomy.

Progressive Approximations in Mass Modeling

That rotation curves become flat at large radii is one of the most famous results in extragalactic astronomy. This had been established by Vera Rubin and her collaborators by the late 1970s. There were a few earlier anecdotal cases to this effect, but these seemed like mild curiosities until Rubin showed that the same thing was true over and over again for a hundred spiral galaxies.

Rotation curves: still flat after a million light-years

This is an update to a post from a few years ago, which itself was an update to a webpage I wrote in 2008, with many updates in between. At that time, the goalposts for detecting WIMPs had already moved repeatedly. I felt some need then to write down a brief synopsis of the history of a beloved hypothesis (including by myself) that had obviously failed as the goalposts were in motion again. That was sixteen years ago.

Updated WIMP Exclusion Diagram

Perhaps the most compelling astronomical phenomenon accessible to a naked-eye observer is a total eclipse of the sun. These rare events have always fascinated us, and often terrified us. It is abnormal and disturbing for the sun to be blotted from the sky! A solar eclipse will occur on Monday, 8 April 2024. A partial eclipse will be visible from nearly every part of North America.

Eclipse Day: 8 April 2024

To start the new year, I provide a link to a discussion I had with Simon White on Phil Halper’s YouTube channel:  In this post I’ll say little that we don’t talk about, but will add some background and mildly amusing anecdotes. I’ll also try addressing the one point of factual disagreement. For the most part, Simon &amp; I entirely agree about the relevant facts; what we’re discussing is the interpretation of those facts.

Discussion of Dark Matter and Modified Gravity

Last time, I expressed extreme disappointment that fossil fuel executives had any role in leading the climate meeting COP28. This is a classic example of putting the the fox in charge of the hen house. The issue is easily summed up:  Setting aside economic self-interest and other human foibles, it is clear from the comments that the science is not as clear to everyone as it is to me. That’s fair;

Global climate basics

Recent results from the third data release (DR3) from Gaia has led to a flurry of papers. Some are good, some are great, some are neither of those. It is apparent from the comments last time that while I’ve kept my pledge to never dumb it down, I have perhaps been assuming more background knowledge on the part of readers than is adequate.

Recent Developments Concerning the Gravitational Potential of the Milky Way. I.

Taking a break from galaxies and cosmology, I’d like to post a little praise of NASA for safely returning a piece of an asteroid to Earth. One of the amazing things to me about astronomy &amp; astrophysics is that we have learned how to decipher the composition of distant stars and gas clouds by observing their spectra. I worked on this early in my career and retain an interest in the cosmic abundance of the elements.

OSIRIS-REx returns safely

It is common to come across statements like “There is overwhelming astrophysical and cosmological evidence that most of the matter in our Universe is dark matter.” This is a gross oversimplification. The astronomical data that indicates the existence of acceleration discrepancies also test the ideas we come up with to explain them.

Checking in on Troubles with Dark Matter

I’m back from the meeting in St. Andrews, and am mostly recovered from the jet lag and the hiking (it was hot and sunny, we did not pack for that!) and the driving on single-track roads like Mr. Toad. The A835 north from Ullapool provides some spectacular mountain views, but the A837 through Rosehall is more perilous carnival attraction than well-planned means of conveyance. As expected, the most contentious issue was that of wide binaries.

The MOND at 40 conference

I recently traveled to my first international meeting since the Covid pandemic began. It was good to be out in the world again. It also served as an excellent reminder of the importance of in-person interactions. On-line interactions are not an adequate substitute. I’d like to be able to recount all that I learned there, but it is too much. This post will touch on one of the much-discussed topics, our own Milky Way Galaxy.

A few words about the Milky Way

I am primarily an extragalactic astronomer – someone who studies galaxies outside our own. Our home Galaxy is a subject in its own right. Naturally, I became curious how the Milky Way appeared in the light of the systematic behaviors we have learned from external galaxies. I first wrote a paper about it in 2008;

Recent Developments Concerning the Gravitational Potential of the Milky Way. III. A Closer Look at the RAR Model

Something that Sabine Hossenfelder noted recently on Twitter resonated with me:  This is a very real problem in academia, and I don’t doubt that it is a common feature of many human endeavors. Part of it is just that people don’t know enough to know what they don’t know. That is to say, so much has been written that it can be hard to find the right reference to put any given fever dream promptly to never-ending sleep.

Let’s just ignore it

Why does MOND get any predictions right?

That’s the question of the year, and perhaps of the century. I’ve been asking it since before this century began, and I have yet to hear a satisfactory answer.

Question of the Year (and a challenge)

We are visual animals. What we see informs our perception of the world, so it often helps to make a sketch to help conceptualize difficult material. When first confronted with MOND phenomenology in galaxies that I had been sure were dark matter dominated, I made a sketch to help organize my thoughts.

Artistic license with the dark matter tree

I noted last time that in the rush to analyze the first of the JWST data, that “some of these candidate high redshift galaxies will fall by the wayside.” As Maurice Aabe notes in the comments there, this has already happened. I was concerned because of previous work with Jay Franck in which we found that photometric redshifts were simply not adequately precise to identify the clusters and protoclusters we were looking for.

By the wayside

The dominant paradigm for dark matter has long been the weakly interacting massive particle (WIMP). WIMPs are hypothetical particles motivated by supersymmetry. This is well-posed scientific hypothesis insofar as it makes a testable prediction: the cold dark matter thought to dominate the cosmic mass budget should be composed of a particle with a mass in the neighborhood of 100 GeV that interacts via the weak nuclear force – hence the name.

The Angel Particle

OK, basic review is over. Shit’s gonna get real. Here I give a short recounting of the primary reason I came to doubt the dark matter paradigm. This is entirely conventional – my concern about the viability of dark matter is a contradiction within its own context. It had nothing to do with MOND, which I was blissfully ignorant of when I ran head-long into this problem in 1994.

Two Hypotheses

I went on a bit of a twitter bender yesterday about the early claims about high mass galaxies at high redshift, which went on long enough I thought I should share it here.  For those watching the astro community freak out about bright, high redshift galaxies being detected by JWST, some historical context in an amusing anecdote…  The 1998 October conference was titled “After the dark ages, when galaxies were young (the universe at 2 &lt; z &lt;

Triton Station